I believe there is NO federal law WRT fraudulent voting. The only voting that is conducted by the federal government is the Electoral College. All other voting is at state or local level.
So, if the fraudulent voting involves identity theft, such as registering dead people, the the FBI would (should) investigate.
However, if some poll workers are able to stuff their ballot box, or manipulate the numbers, then there is nothing the FBI can do. Prosecution would have to be at the State level. (Please note, this is my opinion/understanding. If anyone in Freeper land knows otherwise, please chime in.)
What I am unsure of is whether an alien (illegal or legal) registering as a citizen would be a federal crime. Once again, if anyone knows, please inform us all.
I think the federal government has no business meddling in state or local elections.
BUT when there is a federal office on the ballot, that should give the feds the authority and the responsibility to monitor those elections for fraud of any stripe.